With continuous developments of the wireless cellular mobile communicating system, numbers of mobile device users increase gradually. The rapid development brings opportunities as well as pressure and challenges. A novel communicating technology is required to address many problems such as limited spectrum resources, explosive traffic, excessive load of base station, insufficient coverage and the like. The Long Term Evolution (LTE) system of the 3rd Generation Partnership Project (3GPP) improves structures of the network and proposes a heterogeneous network technology, and further proposes many practical technical solutions.
Direct-connect communications or Device-to-device (D2D) communications are well-known and widely used in various prior wireless technologies, for example an ad hoc multiple-hop self-organizing network. Recently, a cell-controlled D2D communication mode is proposed in the LTE/LTE-Advanced (LTE-A) of 3GPP and is a supplement to the existing cellular network communication. In certain conditions, the D2D communication mode can help base stations to share the communication pressure, divert the communication traffic, ensure the communication quality and enlarge the coverage of the communication, and also can be considered as a kind of heterogeneous network.
In accordance with the spectrum sharing scheme, the D2D communication may be classified into two categories, i.e. an in-band D2D using the spectrum of the cellular network and an out-of-band D2D developing and utilizing the unlicensed spectrum. For the in-band D2D, a key question is to control interferences between D2D users and cellular subscribers. For the out-of-band D2D, a key question is to ensure high quality can be obtained at the unlicensed frequency band. To further classify the in-band D2D, one in-band D2D mode is referred to as an overlay mode, if orthogonal cellular spectrums are used in the D2D link and the cellular link; otherwise, one in-band D2D mode is referred to as an underlay mode, if the D2D link may access the same spectrum as that used in the cellular link. The advantage of the overlay mode resides in that there is no interference between the D2D communication and the cellular communication, so that both of the D2D communication and the cellular communication can obtain high link quality. However, the network has to determine the frequency resources that may be allocated to the D2D communication, possibly reducing the utilization efficiency of the spectrum. In contrast, in the underlay mode, the throughput of the network will be elevated, but complicated interference control mechanisms are required to ensure the service quality.